A concise definition of mulling would read “an intensive and controlled mixing process.” Mulling action is a combination of controlled pressure and agitation, to achieve a high degree of uniformity in the preparation of two or more dry/solid or wetted/solid materials rather than agitation alone.

A mixer for combining solid materials usually operates with a stirring or beating action effected by blades, paddles, arms, or flighting. The normal action of this type of mixer though adequate for simple blending tasks, is not suitable where tempering action, strict uniformity or the intimate blending of a relatively small amount of material into a large mass, is desirable.

The muller is used where materials of disproportionate mass, density, viscosity or particle size must be evenly dispersed to form a homogenous mass… without segregation/or where a small amount of one or more materials must coats each grain of a larger mass.